The present invention relates generally to a fastening element insertable into a hole of a structure, for fastening an object to the structure.
The foregoing fastening element is the type having a shaft that has a smaller cross-sectional thickness along a portion of its length by the forward end than that by the rear end, an expandable sleeve which is fitted over that smaller portion of the shaft, means for holding an object at the rear end of the shaft, and means for expanding the sleeve at the forward end of the shaft.
Fastening is accomplished by inserting the fastening element into a hole and then driving the sleeve onto an expansion element, such as an expansion cone, to tighten the hold.
In the case of known fastening elements, a shaft is fitted with an expandable sleeve over that portion of the shaft having the smaller cross-section and an expander cone is provided at the end of that shaft portion. The expandable sleeve, however, is in the form of two shell-segments, which are connected together on the shaft portion. Alternatively, the expandable sleeve is continuously slotted, which, for the purpose of fitting onto a shaft, is bent open to an appropriate width and bent together again.
In addition, the prior art requires that the expandable sleeve be anchored initially in the drilled hole of an anchoring substrate so that an expander cone can thereafter penetrate into the sleeve and thereby enlarge it when a tensile force acts on the expander cone. Protrusions are arranged on the expandable sleeve to project beyond the diameter of the sleeve so as to press into the drilled hole and provide the necessary anchoring. This manner of attachment of the expandable sleeve into the drilled hole renders the fastening element very sensitive to drilled hole tolerances.
Problems have arisen with both small and large bore drilled holes. In the case of small bore drilled holes, the protrusions press the expandable sleeve so securely into the smaller cross-sectional portion of the shaft that penetration of the expandable sleeve by the cone is, in certain circumstances, no longer possible. In the case of large bore drilled holes, the fastening of the expandable sleeve in the drilled hole may be so slight that, when a load is applied, the sleeve is moved out of the drilled hole together with the cone. The same problem occurs when an initially suitable drilled hole subsequently enlarges because of a crack breaking into the bore. The formation of cracks must be taken into consideration, especially to make the fastening element suitable for the tension zone.
Furthermore, expandable sleeves of soft steel are used in known fastening elements, so that an inelastic expansion occurs in these sleeves under tensile loading. As a consequence, no adjustment of the expandable sleeve can take place, which may result in a loosening and a loss in holding capacity when conditions change, such as when there is a formation of cracks in the region of the drilled hole or when there are loading changes.